Organic acids are valuable products in their own right as food and feed ingredients, for example, or as intermediates. Organic acids can be converted chemically to alcohols, which can subsequently be converted to olefins. Such a process could be envisioned as the basis for a biorefinery to convert biomass resources into a range of products for the energy and chemical industries. Organic acids can be produced by fermentation at very high carbon yield from a wide range of biomass resources.
Many valuable carboxylic acids such as acetic, lactic and propionic acids can be produced by fermentation. Holten, Lactic Acid: Properties and Chemistry of Lactic Acid and Derivatives, Verlag Chemie, 1971; Benninga, (1990), A History of Lactic Acid Making: A Chapter in the History of Biotechnology, Kluwer Academic Publishers, London; Partin, L., Heise, W. (1993), in Acetic Acid and Its Derivatives, Agreda, V., Zoeller, J., ed., Marcel Dekker, New York, pp. 3-13; Playne, 1985 Propionic and butyric acids pp. 731-759, In M. Moo-Young (ed.) Comprehensive Biotechnology, vol. 3, Pergamon, Oxford. However, today almost all carboxylic acids are produced from petrochemicals.
One reason why fermentation routes have failed to compete is that the microorganisms used to produce these acids are inhibited by low pH. In order to achieve high yields, the pH of the fermentation step has to be kept near neutral by the addition of a base such as ammonia, sodium hydroxide or calcium hydroxide. In addition, even at neutral pH, the acids generally inhibit the growth of the organisms used in the fermentation and limit the broth to low concentrations of the acid salt. Thus, the fermentation routes typically produce a dilute aqueous solution of the organic acid salt rather than the organic acid in its protonated form. The salts are highly water-soluble, have a negligible vapor pressure and the carbonyl group is unreactive. These properties make recovery of the salt difficult since distillation, extraction, crystallization and other common industrial separation methods for large-scale production are either technically or economically infeasible.
One way to ease the recovery of organic acid salts is to add a mineral acid to lower the pH of the broth, thereby converting the organic acid into its protonated form. In its protonated form, the organic acid can be more easily recovered by known means such as distillation, extraction or reactive separation processes. Direct acidification with a mineral acid is usually regarded as a troublesome option for recovery of organic acid salts because a salt byproduct is inevitably formed. This byproduct is often of very low value. For example, gypsum is historically the salt co-produced in lactic acid production. (Holten, 1971, Benninga, 1990)Calcium lactate+sulfuric acid=>calcium sulfate+lactic acid
Markets either have to be found or an environmentally responsible disposal method has to be identified. Because of these limitations, much research has gone into alternative methods to recover organic acids.
Recovery of fermentation-derived acetate has been summarized by Busche (Busche, R. M., “Recovering Chemical Products from Dilute Fermentation Broths”, Biotechnology and Bioengineering Symp. No. 13, p. 597-615, 1983 and co-workers at Du Pont (Busche et al., “Recovery of Acetic Acid from Dilute Acetate Solution”, Biotechnology and Bioengineering Symp. No. 12, p. 249-262, 1982) and by Partin and Heise, 1993.
Once the organic acid has been produced in solution in its protonated form by direct acidification, various means can be used for its recovery from the broth (Othmer, “Acetic Acid Recovery Methods”, Chemical Engineering Progress, Vol. 54, No. 7, July, 1958, Baniel et al., U.S. Pat. No. 4,275,234). For example, solvent extraction of organic acids from dilute solution has been studied in detail. (King, et al., “Solvent Equilibrium for Extraction of Carboxylic Acids from Water”, Journal of Chemical and Engineering Data, Vol. 23, No. 2, 1978). Baniel (U.S. Pat. No. 5,780,276) also mentions the use of enhancers in extraction of amines including small amounts of alcohols. However, Baniel (U.S. Pat. No. 5,780,276) provides processes for the recovery of the small amount (10%) of enhancer alcohol and does not suggest its reaction with the organic acid.
Busche (Busche, 1983) discusses general concepts that apply for recovery of organic chemicals present in dilute fermentation broths. He concludes that distillation is preferred for recovery of species with boiling points lower than water (e.g. ethanol) since the product is distilled overhead. If the species has a boiling point higher than water (e.g. acetic acid, propionic acid, lactic acid), then distillation is not preferred since the energy required to boil water and remove it as the overhead product is excessive. He also surveys other means for recovery of high boilers such as extraction, crystallization, evaporation and electrodialysis.
Although not considered by Busche (Busche, 1983), reactive distillation converts a carboxylic acid (a high boiler) into volatile ester (a low boiler), thus sidestepping the energy penalty associated with boiling water. (Xu and Chuang, “Kinetics of Acetic Acid Esterification over Ion Exchange Catalysts”, Can. J. Chem. Eng., pp. 493-500, Vol. 74, 1996, Scates et al, “Recovery of Acetic Acid from Dilute Aqueous Streams Formed During a Carbonylation Process”, U.S. Pat. No. 5,599,976, Feb. 4, 1997).
Many advances have been made in membrane processes during the last two decades. Crossflow microfilters and ultrafilters are routinely used to clarify broths by removing cell mass and insoluble materials. Further clarification by nanofiltration removes high molecular weight soluble impurities such as proteins and residual carbohydrates, and provides the additional benefit of concentrating the organic acid salt prior to downstream recovery. Bipolar electrodialysis can be used to acidify the broth. A patent assigned to Chronopol, Inc. (Miao, “Method and Apparatus for the Recovery and Purification of Organic Acids”, U.S. Pat. No. 5,681,728, Oct. 28, 1997) gives an example of how to sequence the various membrane units to recover and acidify lactic acid from a fermentation broth. Economics for membrane-based processes are favorable today for high value, low volume products. Scale-up of capital cost is nearly linear, so membrane systems are not always competitive for large-scale production of bio-commodities unless high flux is achieved. This is especially true when considering the more complex membrane processes such as bipolar electrodialysis.
Work at DuPont (Yates, “Removal and Concentration of Lower Molecular Weight Organic Acids From Dilute Solutions”, U.S. Pat. No. 4,282,323, Aug. 4, 1981 and Busche et. al., 1982) discusses the use of carbon dioxide as an acidulant to convert fermentation-derived acetate salts into acetic acid and subsequent solvent extraction. Researchers at Cargill (Baniel et. al., “Lactic Acid Production, Separation, and/or Recovery Process”, U.S. Pat. No. 5,510,526, Apr. 23, 1996) have investigated a related method for recovery of lactate by acidification with CO2 and concurrent extraction with an amine. However, this reaction requires high pressure and produces multiple phases (4) at the same point in the process.
Researchers at CPC International (Urbas, “Recovery of Acetic Acid from a Fermentation Broth”, U.S. Pat. No. 4,405,717, Sep. 20, 1983 and Urbas, “Recovery of Organic Acids from a Fermentation Broth”, U.S. Pat. No. 4,444,881, Apr. 24, 1984) also discuss the use of carbon dioxide as an acidulant. Tributylamine (TBA) is normally immiscible with water, but the tributyl amine:acetic acid complex (TBA:HAc) is water soluble. When a dilute aqueous solution of calcium acetate at near neutral pH is mixed with TBA, and then carbon dioxide is bubbled through the mixture, the following reaction occurs at or near ambient temperatures:Ca(Ac)2+H2O+CO2+2TBA=>2TBA:HAc+CaCO3 
Use of a stoichiometric amount of TBA produces a single aqueous liquid phase containing the tributyl amine:acetic acid complex. The reaction is driven to the right since calcium carbonate precipitates upon formation. The amine must be suitably chosen such that its acid/amine complex is completely water soluble. Urbas (Urbas, U.S. Pat. No. 4,405,717) also mentions dicyclohexyl methyl amine.
In one embodiment of the Urbas process, the aqueous amine complex is extracted into an organic solvent, the solvent is stripped off, and the complex is thermally split apart giving the acetic acid product and regenerating both the solvent and amine for recycle. The amine used in the extraction process must be extractable. Urbas shows that amine:acid complexes that are too water soluble cannot be extracted including trimethyl and triethyl amine acid complexes. For extraction, Urbas teaches the use of low boiling, non-reactive solvents that do not azeotrope with acetic acid, with preference given to chloroform. These are severe limitations on solvent selection. Furthermore, use of chlorinated solvents, such as chloroform, would be problematic at industrial scale.
In another embodiment of the Urbas process, the amine acid complex is concentrated by the removal of the water and then thermally cracked to generate the free acid and regenerate the amine. In general, there have been two approaches to recovery of acid/amine complexes, back extraction into water or an aqueous base, or evaporation of the water and then recovery of the acid. However, water removal is extremely energy intensive as outlined previously.
The thermal regeneration reaction has been found by the present inventors to be difficult in practice, leading to a viscous intractable residue and low yield of acetic acid.
In the Urbas process, the calcium carbonate can be recycled for use as a base for neutralization of the fermentation step. No other salt byproduct is created, and this feature is a significant advantage for this route. The use of calcium carbonate as a base in an organic acid fermentation produces CO2 which can be utilized in the reaction step to form the acid/amine complex and the calcium carbonate, so there is not net production of CO2.
Urbas teaches away from the use of solvents which can form esters with the organic acid. However, the present inventors have found that an ester may be a valuable intermediate or product. For example, the use of an alcohol solvent that potentially forms an ester with the acid can be integrated into an indirect ethanol process for example as disclosed in U.S. Pat. No. 6,509,180 (Verser and Eggeman, to ZeaChem) incorporated herein in its entirety by reference. The ZeaChem process describes the production of ethanol by production of acetic acid, esterification of the acid with an alcohol and subsequent hydrogenation of the ester.
The production of esters of organic acids is well known. Esters have been used as an intermediate in the recovery and purification of organic acids. Methods such as reactive distillation as mentioned previously can be used if the acid is in the protonated form, i.e., the free acid. (Benninga 1990, Scates) Various catalysts have been explored to facilitate this reaction including cationic ion exchange resins, strong mineral acids such as sulfuric, hydrochloric and nitric, and strong organic acids such as methane sulfonic acid or toluene sulfonic acid. (Xu and Chuang, 1996, Filachione et al., “Production of Esters”, U.S. Pat. No. 2,565,487, Aug. 28, 1951)
Since esterification is an equilibrium reaction, the reverse reaction of hydrolysis occurs at the same time as the forward reaction. In most processes either water, or the product ester, are removed continuously during the reaction to drive the reaction in the desired forward direction.
Direct esterification of acid/amine complexes have been reported by several groups (Filachione, 1951, Tung et al, “Sorption and Extraction of Lactic and Succinic Acids at pH>pKa, 2. Regeneration and Process Considerations”, Industrial and Engineering Chemistry, Vol. 33, pgs. 3224-3229, 1994, Sterzel et al, “Preparation of Lactates, U.S. Pat. No. 5,453,365, Sep. 26, 1995). Each of these references reports the esterification of the concentrated complex in which the bulk of the water is removed by evaporation or distillation overhead. Thus, these processes suffer from the energy penalty of vaporizing the water.
The hydrogenation or hydrogenolysis of esters to produce an alcohol from the organic acid moiety and to regenerate the alcohol of the ester is well known. McKee, WO 00/53791 Alcohols produced by hydrogenation of esters can also be converted to an olefin derived from the organic acid moiety. The dehydration of alcohols to olefins has been described (Tsao et al., “Dehydrate Ethanol to Ethylene”, Hydrocarbon Processing, 57(2), p. 133-136, February 1978). The process has been practiced at the commercial scale for ethanol dehydration to ethylene. The process is carried out in a fluidized bed with a phosphoric acid catalyst on an inert support.
Similarly, propionic acid can be produced by fermentation, converted to propanol by hydrogenation of a suitable ester and then dehydrated to propylene. (Playne, 1985)
In addition, various esters can be interconverted by transesterification, by reacting one ester with an excess of a second alcohol to form the ester of the second alcohol. Various processes are known in the art for transesterification such as reactive distillation.
Esters may also be hydrolyzed to regenerate the organic acid and the alcohol. Lactic acid has been recovered and purified by esterification with methanol and subsequent hydrolysis. (Benninga, 1990)